For each region, we estimated the input parameters of our PDR models – the gas density \(n\) , the incident radiation field \(G_0\) , and the line-of-sight depth \(A_{\mathrm {V,los}}\)  – from the SED results and the [C ii] and CO data. For the Eastern Rim of the Orion Nebula, [1]} report a gas density, as estimated from the spatial separation of peak [C ii] and CO(2-1) emission, of \(n\simeq 1\times 10^4\,\mathrm {cm^{-3}}\) , while the density drops to \(n\simeq 2\times 10^3\,\mathrm {cm^{-3}}\)  in the southern Veil Shell, estimated from the dust optical depth using the synthetic extinction curves of [2]} with \(R_{\mathrm {V}}=5.5\) . In the rim of M43, [1]} find \(n\simeq 1\times 10^4\,\mathrm {cm^{-3}}\)  from the [C ii] and CO peak separation, as well. In NGC 1977, the dust optical depth indicates a density of \(n\simeq 2\text{-}5\times 10^3\,\mathrm {cm^{-3}}\) , using a line-of-sight length of 1 pc. The radiation field in the Orion Nebula is estimated from the FIR-distance relation, that is \(G_0\simeq 500 (1\,\mathrm {pc}/d)^2\)  (cf. Fig REF ). The FIR emission toward the molecular background of M43 suggests \(G_0\simeq 2000\) . In case of NGC 1977, we have used the stellar parameters of 42 Orionis to estimate the radiation field. In most of the shell, \(G_0\simeq 100\text{-}200\) , in concordance with [4]}. The radiation field in OMC3 may be slightly higher, \(G_0\sim 400\text{-}1000\) ; [5]} estimate \(G_0\simeq 360\) .
